This invention relates to the field of air conditioning, and more specifically to the field of retrofitting existing air conditioning devices with more effective devices to improve occupant comfort and take advantage of inherent diversity in air conditioning loads.
The perimeter induction unit is a type of air conditioning device that was widely used in non-residential building construction during the period 1950-1975. It should be noted that the term "air conditioning" is used herein in its general sense, encompassing all methods of treating air to obtain desired temperature and humidity levels, including cooling, heating and humidification processes. The perimeter induction unit is described in detail below in connection with FIG. 1, and for the present it suffices to describe this unit as an effort to reduce existing air conditioning construction costs in large buildings by performing a portion of the final air conditioning in the space to be conditioned, rather than centrally. Thus, instead of heating or cooling air centrally and distributing it in relatively large ducts, this system supplies primary air at relatively high pressure in small ducts, together with heated or chilled secondary liquid, so that air from the space could be conditioned as required to achieve to desired temperature and humidity. The result was the units found mounted on the floor at the outer wall of many offices, hotel rooms and the like, easily identified by their intake grilles located adjacent the floor and outlet grilles located in the upper horizontal surface. Most such systems were installed as "two-pipe" systems, having a single liquid supply line and a single liquid return line, shared by the chilled and heated water systems, which of course cannot operate concurrently. This system is described in detail in a number of references, and is well summarized in the 1987 ASHRAE Handbook: Heating Ventilating and Air-Conditioning Systems and Applications, published by the American Society of Heating, Refrigerating and Air Conditioning Engineers, at pages 3.1-3.3.
Although the perimeter induction unit did achieve decreased construction costs, it has presented significant problems in service. First, the savings in HVAC system installation is offset by space lost in the heated space, owing to the conditioning unit itself, generally mounted on the floor. This space amounts to about 5-8% of usable floor space in a normal office. Also, this system is particularly unsuited for buildings where large variances in thermal load are experienced. Night building heat loss, coupled with large solar gains require the air conditioning system to be able to provide heating and cooling in the same occupied period. This situation is most prevalent in the early spring and late fall. Unlike buildings using a distributed air system, even zoning the building will not allow for simultaneous heating and cooling of different zones, as the secondary liquid system must be either heated or chilled, not both.
Perhaps most seriously, however, perimeter induction units are inherently susceptible to problems arising during normal use, such as the "bookshelf" effect. The unit is generally about as high as a desk-top or window sill, and the availability of that much flat space in most offices is a magnet for books, papers and the like. These articles cover the outlet grilles, leading to user complaints about poor heating or cooling. Similarly, the secondary heating or cooling that takes place in the unit itself is accomplished by a coil, which is generally protected by a filter. This imposes a maintenance problem greatly exceeding the task of cleaning centrally-located filters, and any lack of such maintenance is soon felt in decreased system effectiveness. Additionally, performance problems may be addressed by inexperience maintenance personnel, or by users themselves, by simply opening secondary liquid flow valves or lowering secondary water temperature in a misguided attempt to improve performance. In fact, such adjustment can produce condensation on the secondary liquid coil, and if the system is not equipped with a condensate removal system (as many are not) then damage to the building rapidly ensues.
Given their many problems, one would expect building owners to abandon perimeter induction units altogether, in favor of modern variable air volume systems. Unfortunately, however, such conversions are not so simple. Primarily, the original justification for perimeter induction units was their small duct requirements, and replacing the entire duct system in an office building is not an attractive option. Structural constraints in vertical ducting and primary equipment spaces virtually eliminate this option. Similarly, some owners have tried to implement a conversion on a floor-by-floor basis rather than installing a central system, but that design requires a mechanical room on each floor, coupled with local duct work and air handling units. Therefore, the expected space gains from such a conversion quickly disappear.
What the art needs, but has not been able to devise, is a way to employ the basic design of the perimeter induction unit system while replacing the units themselves, without requiring replacement of the entire building distribution system. That system is offered in the present invention.